Yarn heating members may be heated indirectly by an electric heating element using a heating fluid as intermediate. The heating fluid will establish a saturated vapour phase under operating conditions so that any cooling of the yarn heating member is followed immediately by cooling and local condensation with an attendant release of heat. Yarn heaters may be of a generally elongate shape and be used in arrays with individual heaters arranged upright. High yarn speeds are possible if the heaters heat the yarns over a sufficiently long path.
Difficulties arise in applying yarn heating members heated by heating fluid to elongate upright yarn heaters. Temperature variations may occur at the top and bottom of the yarn heaters. The temperature may vary to give a permanent temperature gradient or oscillate.
It is the purpose and object of the invention to reduce such temperature variations, without unduly increasing the dimensions of the yarn heaters or reducing the length over which yarn can be controllably heated.